<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const f1 = function () {
        console.log("f1");
      };

      function say(a) {
        // let a = f1; 回调函数a
        a(); // f1();
      }
      say(f1);
    </script>
  </body>
</html>
